In-app purchase enabled, but not working

In such a situation, if you're sure that in-app purchases are not restricted, I would contact Apple Support using this link and choosing country, Apple ID, then iTunes&AppStore:--> iTunes Store & App Store – Get support for various issues

If your problem is not listed, tap/click on the last option “the topic is not listed” and describe your issue.

Regards

Giulio

This is how you may --> Buy additional app features with in-app purchases and ...

But not --> If your payment method is declined in the App Store or iTunes ...